Typically, rotary wing aircrafts like helicopters are sustained by a rotor, rotating about a vertical rotor shaft, generating lift or upward thrust. The direction of thrust is perpendicular to the rotating plane defined by the path the tip of the rotor blades follows when they rotate about the rotor shaft.
In a conventional helicopter the total thrust from the rotor can be changed by collectively changing the pitch angle (or in short; the pitch) of all the rotor blades. The pitch is in the field of propeller aero dynamics defined as the lateral angle between the blades and a reference plane perpendicular to the rotor shaft axis. The angle is measured tangential to the rotation and will therefore not change if the rotor is tilted.
By collectively changing the pitch of the rotor blades the helicopter can be controlled in the vertical direction. The horizontal direction of flight may be altered by cyclically adjusting the pitch of the blades. Cyclically adjusting the pitch, means that the pitch of individual rotor blades are adjusted from a maximum in a particular position of rotation to a minimum at the opposite side. This causes the lift in one part of the rotation to be larger than in other parts, whereby the rotating plane is tilted with respect to the rotor shaft axis. When the rotating plane is tilted like this, the initially vertical thrust also tilts, and therefore gets a horizontal component, pulling the helicopter in the direction of the downwardly tilted rotating plane.
Normally, a helicopter must be actively controlled either by a pilot or from gyroscopic sensors. The necessary means to varying and controlling the pitch angle of each blade is complicated, expensive and add weight to the helicopter.
A fixed pitch rotor without individual blade control would enable a simpler and more light weight helicopter or aircraft. However, a fixed pitch rotor is inherently unstable in hover (remaining stationary in the air) and requires other means of control. There are several examples of helicopters with fixed pitch rotors including fixed pitch counter-rotating rotors, controlled by some kind of weight shifting.
U.S. Pat. No. 6,182,923 discloses a helicopter where the rotor assembly is able to slide in the longitudinal direction of the fuselage, and at the same time it is able to tilt in the transverse direction. The purpose of this arrangement being to alter the center of gravity relative to the thrust from the rotor, thereby tilting the helicopter in the desired direction of flight. Another fixed pitch counter-rotating coaxial rotor helicopter is disclosed in U.S. Pat. No. 6,460,802. In this helicopter the rotor assembly including the engine is pivotally connected to the fuselage and can tilt in any direction, thereby controlling the aircraft.
In several helicopters designed and built by Kaman Corporation, the inner part of the rotor blades have a fixed pitch similar to the rotors above, while they can twist in the longitudinal direction. On the Kaman helicopters the rotor blades are actively controlled by “servo flaps” adapted to twist the blades in order to cyclically change the pitch, thereby controlling the direction of flight.
Control of an aircraft with a fixed pitch rotor can also be achieved by operating vents or slots to alter the flow of air going through the rotor. Another alternative is to use 4 separate propellers, 2 and 2 rotating in opposite direction and placed diagonally about a central vertical axis. Each propeller initially producing an equal part of the vertical thrust needed to lift the aircraft. The aircraft is controlled by tilting it in the desired direction of flight by increasing the thrust from a propeller on one side of the aircraft and reducing the thrust from the propeller on the opposite side. This idea was first realized in a full scale aircraft in 1920. A similar but very small toy aircraft, battery powered and remotely operated, was introduced by Keyence Corporation, Japan in 1997.
The aircrafts described above are examples of simple designs, however, they are not passively stable and therefore need to be controlled by an experienced pilot or operator. Most of them are operated under light wind conditions or indoors.
A rotary wing toy aircraft passively stable in hover is disclosed in U.S. Pat. No. 5,297,759. This aircraft is in fact a large flying rotor with limited possibilities for control.
Another stable toy helicopter is disclosed in U.S. Pat. No. 6,659,395. This patent uses the word propeller or main propeller in stead of rotor, also when it refers to helicopters. Here a helicopter utilizes different kinds of rings or safety arcs attached to the tip of the propeller blades and it relies on gyroscopic forces to change the pitch of the whole propeller to secure stability, much like ordinary 2-blade rotors with large stabilizer bars. The purpose of the safety arcs, apart from making the propeller more safe is described to be: When the main propeller rotates, if the main propeller begins to pitch (tilt about the longitudinal axis of the blades), the safety arcs will begin to move of the horizontal plane. The weight of the safety arcs however, create a gyroscopic effect causing the main propeller to level out by pivoting the blades about a pivot pin with a pivot axis parallel to the blades. This secures that the propeller remains in, or returns to, a horizontal level. In this helicopter, the propeller blades extend outwards from the shaft in a horizontal plane without any coning, and the blades are free to pivot in a way that changes the pitch along the whole propeller without any twisting or bending of the blades.
In the preferred embodiment of this toy helicopter the propeller is actually prevented from flapping (tilting up and down). The purpose of this seems to be to effectively keep the propeller level at all times, thus preventing the helicopter from going into oscillations or becoming unstable. However, this stabilizing system gives limited possibilities for control in the horizontal direction and does not allow for precise maneuvers. This is partly due to the fact that the propeller and the helicopter acts as an common gyroscopic system resisting any attempts to tilt it. It is believed that if a tilting force was applied to the helicopter for a period of time, starting a horizontal movement, it would take a equal and substantively long time to stop the helicopter, making precise maneuvers difficult.
An other problem with a rotor (propeller) like the one described above, having blades not being able to flap, is that if the helicopter actually was tilted by applying an external force, the rotor, due to its inherent weight and gyroscopic effects, would tilt sideways. The sideways tilting could again give rise to new mechanical forces and cause the rotor to tilt in yet another direction. The rotor could then, despite of its stabilizing means, come out of control and the helicopter loose its stability.
In alternative rotor designs disclosed in U.S. Pat. No. 6,659,395 discussed above, circular rings, similar to those used in toy helicopters for decades, are connected to the tip of the rotor blades via pivots or to the rotor center via fly bars. These rotors are functioning more or less along the same principles as described above, trying to maintain a horizontal level at all times. They are believed to have the same limitations as the preferred embodiment.
In many situations and applications it would be desirable to have an aircraft that is stable without any form of active stabilization, even if high forward speed and the ability to operate under windy conditions is sacrificed.
On this background it can be appreciated that there is a need for a rotor that enables this kind of stability. The rotor should also be able to passively keep an aircraft in a fixed position with respect to the surrounding air. Finally, the rotor should allow for full horizontal control and make precise maneuvers possible.